I think I would be ok to make it public, but in general it would be good to rethink some of the ultratb code, it could take the style as a configuration option in the constructor, and could use a lot of new CPython new features like actually use annotations, and also implement the "did you mean" when there are typoes.

In general it is also emitting directly ascii code; in a perfect world I'd like to return more structered data at at least a number of token, style pairs so that rendering could be better handled on a per-frontend basis.

Even in html we could make use of hover to see variable values; or collapse frames.

But all that needs time, and I barely have a few hours a month on IPython, so no no short term plan to change.

If you send a PR that make this value public (with I guess a property to warn of deprecation of the private attribute I'm happy to merge.